A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer), Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a known lithography system, the projection system comprises a vacuum chamber wherein the patterned beam of radiation is projected. As such, the projected beam traverses at least a region of the vacuum chamber. In the vacuum chamber, contamination with particles, such as carbon hydroxyl particles originating from substrate structures, might damage an optical element, such as a mirror. Further, the contamination particles might influence the optical transmittance of the projected beam. Especially, an extreme ultraviolet (EUV) lithography system might suffer from such gas contamination.
In U.S. patent application publication no. U.S. Pat. No. 6,714,279, a vacuum chamber is provided with an inert gas supply. By supplying an inert gas in the vacuum chamber contamination of optical components may be suppressed.